Single-walled carbon nanotubes are thought to be the ideal candidate materials for next-generation nanoelectronics, sensing devices and optoelectronics. A key stumbling block is the contamination from metallic (met-) SWNTs, which significantly affect device performance and yield by shorting the source and drain electrodes. It is a daunting challenge to effectively remove those met-SWNTs since all known nanotube synthesis methods produce mixtures of semiconducting (semi-) and met-SWNTs and the differences in most of their physical and chemical properties are subtle. In the past decade, numerous sorting methods have been explored. Some methods exploit the significant different electronic properties. The metallic-selective electrical breakdown methods exploit the vastly different intrinsic electronic transport properties between species but these techniques require complex transistor gating structure which is difficult to implement realistically. Various chirality-selective charge transfer chemical reactions have been shown to be highly sensitive to differences in the chirality-dependent nanotube electronic density of states but have not demonstrated high sorting purity. Some possible causes are other competing tube-tube interactions such as strong tube-tube interaction that cause bundling, solvent-tube interaction, and insensitivity of chosen physical technique (e.g. electrophoresis or centrifugation) which probe the indirect properties (e.g. mobility or mass) of the reaction product.
Another class of techniques exploits the selective binding of certain surfactants/polymers/DNA on semiconducting nanotubes and these utilize the indirect subtle structural differences between the nanotube species. Various recently demonstrated sorting techniques in this class such as Density Gradient Ultracentrifugation (DGU) and column chromatography (CC) are based on the same anionic co-surfactant system of sodium dodecyl sulphate (SDS) and sodium cholate (SC). The surfactants have been thought to aggregate differentially around met-SWNTs versus semi-SWNTs due to their different polarizabilities. These biomolecules also function as dispersing agents to suspend the nanotubes as individual species that otherwise bundle to prevent sorting. Although these bulk methods involving dispersing agents are convenient, the sorting selectivity (of around 95%) may still be insufficient. Multiple DGU cycles or multiple may be required to pass through the column to achieve around 99% purity, which may lower the throughput and increase cost. All these existing bulk sorting methods are based on subtle and indirect differences between the metallic and semiconducting nanotubes and such small indirect differences between nanotube species may make it difficult to achieve high throughput and high purity needed. A purity of around 99.99% may be required for nanoelectronics.